This invention relates generally to rolling element jackscrews designed for converting a rotary input to a linear output with lower internal friction. More particularly, this invention relates to an improved rolling element jackscrew of simplified design which provides a low lead precision linear output with a high mechanical advantage.
Rolling element jackcrews in general are well known of the type including a plurality of elongated threaded planetary rollers meshed between a threaded shaft and a threaded nut. In such devices, the planetary rollers are designed to orbit the shaft while simultaneously rolling about their own axes in response to rotary input driving of either the shaft or the nut. This combined rolling and orbiting motion of the planetary rollers transfers the rotary input from the shaft to the nut, or vice versa, to provide a linear jackscrew output. Advantageously, the rolling action of the planetary roller provides a relatively low friction mechanical coupling between the shaft and nut thereby providing a relatively high efficiency motion conversion device. Rolling element jackscrews of this general type have found widespread use throughout the mechanical arts.
Previous rolling element jackscrews, however, have exhibited a number of disadvantages which have effectively limited their practical utilization. More particularly, many prior jackscrews have been proposed with component thread formation requiring at least some axial migration of the planetary rollers during operation, wherein such migration effectively limits the magnitude of the linear output stroke. In some jackscrew designs, relatively complex mechanisms have been proposed for recirculating the migratory rollers, such as depicted in U.S. Pat. No. 3,884,090, but these jackscrew designs have been undesirably costly and unduly complex while presenting a varying internal torque loading as the rollers move in and out of engagement therewith.
Still other rolling element jackscrew designs have been proposed with the various components having thread forms of a common pitch intended to limit or prevent undesired axial migration of the planetary rollers. However, the use of commonly pitched threads on the shaft and nut limits the position of roller installation to a single circumferential position about the components for each spiral thread, since the commonly pitched threads axially align at only one circumferential position. Since a minimum of three rollers are required to support the shaft within the bore of the nut, at least three thread starts are thus inherently required whenever common thread pitches are used. Unfortunately, the use of multiple or multistart threads substantially increases the linear output stroke in response to a single input rotation to correspondingly decrease the mechanical advantage between the rotary input and the linear output. Moreover, when three or more threads are used, it has been recognized that undesired axial and/or rotational roller slippage can occur, wherein such slippage has in the past been prevented by adding components such as synchronizing gears which undesirably increase jackscrew cost and complexity.
There exists, therefore, a significant need for an improved and mechanically simplified rolling element jackscrew which avoids axially migrating planetary rollers and thus also avoids use of complex roller recirculating mechanisms. Moreover, there exists a significant need for an improved jackscrew designed to substantially prevent axial or rotational roller slippage without requiring use of synchronizing gear components, and which further exhibits a substantially optimized mechanical advantage and load-carrying capacity. The present invention fulfills these needs and provides further related advantages.